1. Field of the Invention
This invention pertains to the treatment of waste liquids by, for example, neutralization, oxidation, or flocculation.
2. Description of the Prior Art
To reduce water pollution of streams, lakes, and other water sources, a large number of industrial and municipal waste waters require some sort of treatment prior to discharge. The waste liquids often have an imbalance in ion activity, causing them to be either acid or basic in varying degrees. The ion imbalance usually must be substantially removed to satisfy conditions for acceptable discharge.
The amount of acidity, alkalinity, or other ion imbalance in the waste stream must be measured and treated to assure proper neutralization and destruction or conversion of harmful ion concentrations so that waste water discharges are environmentally acceptable and harmless to aquatic life and to other subsequent users of the water. Often the waste liquid streams have variable flow rates and variable compositions which can result in fluctuations of quality and quantity of effluent discharge.
To assure a reliable and thorough treatment of waste liquids under the various inlet conditions it is known to be necessary to provide appropriate sensing and control loop instrumentation which must be capable of indicating the stream concentration parameters and responding with signals to correct any deviation from an established set point.
Treatment of waste liquids can be performed either in batch or continuous systems. For neutralization in batch systems the waste liquid is transferred to a convenient process vessel and stirred while the treating substance is added. The pH or other appropriate physicochemical property is then measured and compared to the value required for discharge. Deviations from this required value are noted and, if necessary, more treating substance is added and mixing continues until the required value has been reached.
In a batch process, the treating substance is added in increments since only after thorough mixing of the total contents of the vessel is it possible to obtain a physicochemical property measurement representative of the entire vessel and to determine whether the amount of treating substance added was correct. Waste of treating substance can be a normal result of batch treatment and the larger the treatment vessel the longer the mixing time required to obtain results of additions of the treating substance. False readings are possible by incomplete mixing which occurs when portions of unmixed treating substance contacts the measuring element; however, if time is not a factor the batch can be mixed long enough to insure desired treatment of the waste liquid.
To overcome the disadvantages of batch treatment, continuous treatment systems which can operate automatically have been developed. The desirable steps in a continuous treatment process are: collection of waste liquid in the treatment vessel; mixing of waste liquid and measurement of the value of the appropriate physicochemical property; corrective action by admission of the required treating substance, such as an acid or a base; mixing of the treating substance with the contents of the vessel; measurement of the physicochemical property after mixing; and discharge of the treated waste liquid. All of these steps have to take place within a very short time to allow the process to be continuous; the physical layout of components makes this difficult to achieve.
In a conventional continuous system for waste liquid treatment a vessel is usually used with a mechanically driven propeller mixer to agitate and provide mixing of the waste liquid with the treating substance. Such mixing vessels have very good local mixing properties but the total vessel volume must be mixed to obtain homogeneity. The retention time of individual molecules varies and many molecules which entered with the waste liquid may be discharged from the vessel without having been exposed to the treating substance. Some system designs change the position of liquid entry from the top of the vessel to the bottom of the vessel or provide for several mixing vessels in series, all in an effort to improve effluent quality.